Also, improve reporting of version resolution. e.g.:
```
$ ./emsdk install sdk-latest
Resolving SDK alias 'latest' to '2.0.23'
Resolving SDK version '2.0.23' to 'sdk-releases-upstream-77b065ace39e6ab21446e13f92897f956c80476a-64bit'
Installing SDK 'sdk-releases-upstream-77b065ace39e6ab21446e13f92897f956c80476a-64bit'..
...
```
Previously this had to be
emsdk install sdk-releases-upstream-HASH
The only thing preventing using just the hash was that there was no
default for the backend, so defaulting to upstream fixes this. And then
we can do
emsdk install HASH
